1. Field of the Invention
This invention relates to electron gun design and more specifically to a technique for combining the benefits of a large main lens and an Einzel gun's dual-lens structure in a low voltage high-resolution Einzel gun.
2. Description of the Related Art
A CRT type electron gun is comprised of two or more optical parts; the triode and one or more focusing lenses. The triode is made up of the Emitter (cathode), the Wehnelt suppressor electrode (biasing grid) and the extractor electrode (first accelerator grid). The focusing lenses include a pre-focusing lens and one or more main lenses.
In the triode, the cathode's emission current is subject to two limitations. First, the cathode has a temperature limited emission current density, which varies widely from cathode to cathode. Differences in the cathode's activation and the tube's vacuum quality can change the temperature limit of the current density. Secondly, the cathode emission is subject to a space charge limitation at the surface of the cathode, which is determined by the physical geometry of the triode. Typically, the triodes are designed to operate in space charge limited conditions because physical geometry is more consistent and easier to control than the thermal emission properties of the cathode.
Under space charge limited conditions, the cathode is heated to a temperature that causes electrons to be emitted at the cathode surface. The electrons are then pushed back to the cathode surface by the suppressor electrode. But, the suppressor electrode has an optical aperture that allows an extraction voltage from the first accelerator to penetrate through the aperture and strip electrons off of the cathode. This structure produces a converging electron beam that crosses over at an axial position somewhere between the biasing grid and the first accelerator, typically referred to as the “first crossover”.
The biasing grid effectively forms an iris, which the beam passes through. This iris can be opened or closed by varying the voltage on the biasing grid. If the biasing voltage is brought closer to the cathode voltage then the cathode's active emitting surface becomes larger in diameter. This active area serves as the object in the total optical system. While this voltage change allows more current to escape from the cathode it increases the object size for the optical system.
Increasing the extraction voltage on the first accelerating grid increases the biasing voltage required to “cutoff” the beam. This causes the active cathode surface to decrease in size but reduces the slope of the current vs. biasing voltage curve. This increase of the extraction voltage also increases the beam angle (increase in convergence before the first crossover or increase in divergence after the first crossover), which could be desirable or undesirable depending on the size of the main focusing lens.
The beam is sent through a series of focusing lenses (pre-focus lens, main lens, etc.) that focus the beam at the target. A lens is formed any time the beam is subjected to a change in the electric field and is typically constructed by sending the beam through two cylindrical shaped grids with differing voltages. The greater the potential difference between the grids the stronger the lensing effect. However, a stronger lens has more spherical aberration. Therefore, splitting the focusing between multi-pole lenses may be desirable.
A large beam is desirable because it has a steeper crossover angle at the first crossover, which reduces the spot size at the target. But, as the beam increases in size the spherical aberration affects increase the spot size. Thus, the Triode must be optimized for the best possible spot size for a given focusing lens system. Maximizing the main lens diameter will reduce spherical aberration.
Electron guns are typically given a name that describes their focusing lenses. A standard bi-potential gun has an Anode voltage and a focus voltage that together define a single focusing lens. As shown in FIG. 1, the standard Einzel gun 10 has a pre-focus lens 12 and two main lenses 14a and 14b. 
Einzel gun 10 includes a second accelerator electrode 16 that follows a triode 18. The volume between the first accelerator electrode and the second accelerator electrode forms pre-focus lens 12. This combination of the triode and pre-focus lens is often referred to as the “Beam Forming Region” or “BFR,” which is followed by the main lens system. In an Einzel gun the main lens system is split in to two main lenses 14a and 14b. The volume between second accelerator electrode 16 and a focus electrode 20 forms first main lens 14a. The volume between the focus electrode and a final accelerator electrode 22 forms second main lens 14b. 
By definition, the second accelerator electrode and final accelerator electrode are both held at anode potential and the focus electrode is at a lower potential. The second accelerator electrode is electrically connected to the final accelerator electrode via a jumper 24. The final accelerator electrode is connected to an internal conductive coating 26 on the inside of the neck glass 28, which is held at anode potential, by a number of snubber springs 30. The diameter of the main lenses is limited to the space between the mounting beads 32a and 32b. The smaller the main lenses the greater the spherical aberration for a given beam size.
The Einzel gun uses a standard 14-position stem 34 of which 8-10 pins are typically used depending upon the placement of mounting beads. Nine low voltage pins 36 (2 filament, 1 cathode, 1-5 suppressor electrode, and 1 extractor electrode) are adjacent one another (only two of which are shown) and 1 high voltage pin 38 for the focus electrode is spaced apart on either side by 2 unused pins positions 40 to prevent arcing. High voltage pin 38 is connected to focus electrode 20 via a lead 42. The second and final accelerator electrodes are connected by internal jumper 24 and connected to anode potential through an anode button (not shown) in the neck.
Einzel guns are particularly well suited for applications that have a short focal length and a large beam deflection angle. Projection tubes, television and monitors operate at anode potentials >20 Kv and are generally considered to be high-voltage applications. Low-voltage applications, anode potential <12 Kv, include helmet mounted displays (HMDs) hand-held displays and displays that use a secondary emission target in place of a phosphor screen.
U.S. Pat. No. 5,894,190 to Hirota teaches a modified high-voltage Einzel Gun of the type shown in FIG. 2, in which the diameter of the main lens 44 has been increased thereby reducing spherical aberrations. Hirota's invention allows an increase to the main lens size by extending both the final accelerator electrode 46 and the focus electrode 48 forward past the end of the mounting beads 50. This allows the electrodes to maintain their standard mountings. For example, Hirota uses a jumper 52 between the second and final accelerator electrodes, which are then connected to anode potential by the anode button in the neck. The electrodes then have an increased diameter in the sections of the electrodes that are past the mounting beads. However, Hirota's Einzel gun still requires the final accelerator electrode to be smaller than the inside diameter of the neck glass.
As shown in FIG. 3, the standard bi-potential gun 54 contains a triode 56 and two additional electrodes. These electrodes are the focus electrode 58 (grid #3) and the final accelerator electrode 60 (grid #4). The volume between the first accelerator and the focus electrode forms a pre-focus lens 62, which in combination with the triode forms the Beam Forming Region. The main lens system is comprised of a single main lens 64, which is formed by the volume between the focus electrode and the final accelerator electrode. The final accelerator electrode is connected to an internal conductive coating 66 on the inside of the neck glass 68, which is held at anode potential, by a number of snubber springs 70. In the standard bi-potential gun the diameter of the main lens is limited to the space between the mounting beads 72.
As shown in FIG. 4, the bi-potential gun can be modified using internal conductive coating 66 on the neck glass 68 as the final accelerator electrode (grid #4). Unlike the Einzel gun, the final accelerator grid is not jumpered to any other electrode and thus can be replaced with the internal conductive coating. This extends the focus lens 64 past the end of the mounting beads 72 and allows the focus electrode 58 to maintain its standard mounting. The focus electrode has an increased diameter in the section of the electrode that is past the mounting beads. In fact, this configuration provides the maximum possible size for the single main lens, which reduces spherical aberrations.
U.S. Pat. No. 4,590,403 discloses another type of gun, the tri-potential gun that uses the internal conductive coating to define the final accelerator electrode, which as in the bi-potential gun is isolated from the other electrodes. Alig's gun has a triode and four more electrodes including the first focus electrode (grid #3), the decelerating electrode (grid #4), the second focus electrode (grid #5) and the final accelerating electrode (grid #6).
The volume between the first accelerator electrode and the first focus electrode forms a pre-focus lens, which in combination with the triode forms the Beam Forming Region. The main lens system is split into three main lenses. The volume between the first focus electrode and the decelerating electrode forms the first main lens. The volume between the decelerating electrode and the second focus electrode forms the second main lens. The volume between the second focus electrode and the final accelerating electrode forms the third main lens. This design improves the pre-focus lens and reduces spherical aberrations as compared to the Einzel gun and bi-potential gun.